When an image is printed by a printer or the like, a bit map that is a two-dimensional arrangement of pixels forming an original image is converted into a bit map with a higher resolution before printing is carried out. When doing so, some devices improve the printed image quality by smoothing corner parts of pixel patterns in the bit map.
Methods of smoothing such corner parts are disclosed by Japanese Laid-Open Patent Publication Nos. H7-283946 and H7-288693, for example. In such conventional methods, pixel patterns of corner parts are detected from the bit map and such pixel patterns are replaced with smoothed pixel patterns given in a look-up table that is prepared in advance. A method that performs smoothing by inserting average values for two lines of pixels in the original image between those two lines is also known.
The smoothing method that inserts pixels with averaged values between two lines of pixels in the original is realized by simple processing and there is no need to prepare a look-up table, so that the method can be used easily. However, in this method, pixels that are arranged for smoothing corner parts are concentrated in the corner parts, so that on the contrary stepped parts actually become more prominent. As a result, in the original image, parts formed of a plurality of consecutive steps so as to represent an oblique outline become wavy, so that there is the problem of linear outlines being lost when the resolution is raised.
A smoothing method that uses a look-up table can produce smoother outlines than the method described above by preparing pixel patterns in advance so that the pixels disposed for smoothing purposes are not concentrated in corner parts. For example, in the Japanese Laid-Open Patent Publication Nos. H7-283946 and H7-288693 mentioned above, smoothing is performed not only by adding pixels to fill in corner parts but also by cutting out parts with consideration to the white/black balance. However, processing time is required to search the look-up table. Additionally, it is not easy to produce a look-up table in advance. There is also a limit on the pixel patterns that can be prepared as a look-up table, which limits the range of corner parts that can be amended. Accordingly, although it is possible to amend short-cycle unevenness extremely well, long-cycle unevenness cannot be amended. This means that when an original image includes a part formed of a plurality of consecutive steps so as to represent an oblique outline as described above, raising the resolution results in a wavy state, again causing the problem of a loss of linear outlines.
It is an object of the present invention to provide a method and apparatus that can perform smoothing without using a look-up table and can also perform smoothing favorably even when there is long-cycle unevenness. It is a further object of the present invention to provide a method and apparatus that can perform smoothing where linear outlines are not lost when raising the resolution of parts formed of a plurality of consecutive steps for representing an oblique outline in an original image.